The pathogenic properties of Salmonella are evaluated in a modified in vitro invasion system employing the HeLa Cell. The validity of this system has been established utilizing fresh isolates of Salmonella from human and animal sources. Specific metabolic defects introduced into the pathogenic Salmonella strains by a transposon have produced apparent avirulent mutants. All pathogenic Salmonella are lysogenized by bacteriophage which we have partially characterized by heat sensitivity and neutralization kinetics to be similar to phage P22. Restriction enzyme fractions obtained by EcoR1, Bam I and Hind II restriction enzymes are similar. The defect appears to retard the attachment or penetration mechanism. Preliminary electron microscopy studies of the intracellular interaction between the bacteria and the host cell implies an activity mediated destruction of the mammalian cell.